Materials for permanent magnets, such as neodymium iron barium (NeFeB) or samarium cobolt (SmCo), with which strong magnetic flux density 1 . . . 1.2T can be achieved, are typically very brittle. Especially the ability of the permanent magnet materials to withstand tensile stress is poor. Thus, permanent magnets in the rotor of a permanent magnet electrical machine usually have to be strengthened with support structures made of more viscous material. The said strengthening is very important especially in permanently magnetised high-speed electric machines, in which the peripheral speed of the outer surface of the rotor can be even hundreds of meters in a second.
FIG. 1a illustrates the state-of-the-art rotor structure for a permanent magnet electrical machine, seen from the side. FIG. 1b illustrates the cross section A-A of the said rotor structure. The rotor structure has a shaft 101 with permanent magnets 102-113 on its outer surface. The shaft is preferably made of ferromagnetic steel. The outer surface of the shaft 101 is provided with a sleeve 115 made of magnetically non-conductive material, with through-holes for the permanent magnets 102-113. The arrows drawn to the permanent magnets 102-113 describe the magnetising direction of each permanent magnet. A support collar 114 has been arranged onto the sleeve 115 and the permanent magnets 102-113. The support collar 114 is shown in longitudinal section in FIG. 1a. The sleeve 115 can be attached to the shaft 101 by, for example, screws 116 and/or a crimped joint based on thermal expansion phenomenon, i.e. thermal crimping. The sleeve 115 can be made, for example, of plastic, aluminium, titanium or some other suitable magnetically non-conductive material. In this document, material, which is not ferromagnetic, is called magnetically non-conductive material. In the rotor structure shown in FIGS. 1a and 1b, each permanent magnet 102-113 is assembled of several axially consecutive pieces. In this document, axial refers to the direction of the rotor's rotation axis 160. FIG. 1c illustrates a longitudinal section of area B shown in FIG. 1a. The longitudinal section plane is the x,y plane marked to FIGS. 1a and 1c. FIG. 1c corresponds to an exemplary situation, in which the rotor structure rotates around the rotation axis 160. Permanent magnets are influenced by centrifugal force, which causes the permanent magnets to be pressed against the support collar 114. In other words, the support collar 114 directs radial forces to the permanent magnets, the forces keeping the permanent magnets on the circular orbit. Forces between the permanent magnets and the support collar stretch the support collar. In FIG. 1c, the stretching of the support collar has been strongly exaggerated in order to demonstrate the phenomenon. FIG. 1c shows how the permanent magnet 106 is pressed against the support collar 114. A strong surface pressure is applied to the area of the permanent magnet 106 illustrated by the arrow 130. The said strong surface pressure stresses the brittle permanent magnet material 106 and may even cause fractures. An adversely high surface pressure may also be directed to the support collar 114. The situation can be somewhat improved by rounding the edge of the permanent magnet 106 indicated by the arrow 130 and/or by selecting the length of the support collar so that the ends of the support collar and permanent magnets are aligned with each other as closely as possible in the axial direction. The realisation of these matters is complicated by the hard machineability of the permanent magnets and the possible moving of the support collar in relation to the permanent magnets.
One state-of-the-art solution is to attach the permanent magnets 102-113 to the shaft 101 so that the pressing of the permanent magnets against the support collar 114 would be lighter. The said attachment is generally done by gluing. However, in high-speed electric machines this solution is generally not feasible, because achieving a sufficiently strong attachment between the brittle permanent magnets 102-113 and the shaft 101 is a very challenging task. On the other hand, if the attachment between the permanent magnets and the shaft were able to endure the stress caused by the centrifugal force, i.e. the said attachment could keep the permanent magnets on the circular orbit, internal tensile stresses would be generated to the permanent magnets. This would not be advantageous, because the ability of several permanent magnet materials to tolerate tensile stress is poor.